Tube-voltmeter



Patented May 9, 1933 UNITED STATES PATENT OFFICE JAMES D. BOOTH, OF LONGMEADOW, MASSACHUSETTS, AS SIGNOB TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION 01' PENNSYLVANIA TUBE-VOLTMETER Application filed July 16, 1929. Serial 110. 378,674.

My invention relates to electrical measuring instruments and more particularly to vacuum-tube voltmeters.

A number of so called tube-voltmeters have been designed and used, but none of them have been capable of measuring true root-mean-square values of voltage, and this is particularly true at audio frequencies. All of the vacuum-tube voltmeters of which I am aware, have been designed with the assumption that the waves encountered are perfectly sinusoidal. Although this assumption may approximate a true condition in the case of radio-frequency waves, it does not hold in the case of audio-frequency conditions, and, in every case of which I am aware, the possibility of harmonics has" either been completely disregarded or it has been admitted that harmonics interfere with the calibration.

It is an object of my invention, therefore, to provide means for accurately measuring the root-mean-square value of an alternating voltage regardless of the wave form or frequency.

A further object of my invention is to provide an inexpensive and accurate vacuumtube voltmeter having a low power consumption.

In accordance with my invention, I have designed a voltmeter which measures RMS voltages correctly regardless of the wave form of the voltage bein measured and, in addition, it is capable o measuring directcurrent voltages regardless of polarity and without alteration or correction in changing from direct-current to alternating-current voltage measurements. The accuracy of my voltmeter is as good as that of the usual portable indicating-type voltmeter and possesses ratus for balancing an input circuit to ground, and

Fig. 3 is a view, similar to Fig. 2 of a modification thereof.

In the drawing, 1 represents a source of voltage to be measured and 2 is an attenuator or potentiometer connected to the voltage source 1 by conductors 3 and 4'. Two thermionic tubes 5 and 6 of the audion type, each comprising an anode or plate 7, a heated filament orelectron-emitting cathode 8 and an auxiliary electrode or grid 9, all sealed in an evacuated vessel 10, are so connected that their plate currents are additive but the voltage placed in series with the grid bias is positive for one tube and negative for the other.

The output circuit of the attenuator 2 is connected, by conductors 11 and 12 to the grids 9 of the tubes 5 and 6, respectively. The filaments 8 of the two tubes are connected to a battery 13 through adjustable resistors 14 and 15 and conductors 16 and 17.

A negative bias is impressed on the grids 9 of the tubes 5 and 6 by a battery 18 having its positive terminal connected to the variable resistor 14 and its negative terminal connected to a potentiometer 19 through a movable contactor 20. The ends of the resistor 21 of the potentiometer 19 are connected, through fixed resistors 22, to the conductors 11 and 12, respectively, leading to the grids ductor 27 connects the other terminal of the micro-ammeter 24 to the negative terminal of the battery 25, and conductors 28 and 29, respectively, connect the positive terminal of the battery 25 to the plates 7 of the tubes 5, and 6.

The conductors 28 and 29 are connected to the battery '25 in such manner that the voltage therefrom to the plates or anodes 7 may be varied or made dissimilar to each other for par oses of adjustment.

lifeans for balancing the input to the tubes 5 and 6 from the attenuator 2 are illustrated in Figs. 2 and 3.

In Fig. 2 a standard lVagner earth connection is illustrated and comprises a resistor 30 connected across the conductors 11 and 12. A movable contactor 31 is connected between the resistor 30 and ground 32 by a conductor 33. A pair of telephone receivers 34 are connected to the contactor 20 by a conductor 35 and to the conductor 33 by a conductor 36.

The position of the contactor 31 of the resistor 30 is adjusted until the telephone receivers are quiet, at which position the currents through the resistors 22 are equal and no stray currents exist between the mid point of the contactor 20 and ground.

A modification of the input balance shown in Fig. 2 is illustrated in Fig. 3. In this modification, the variable resistor 30 has been replaced by a variable condenser having a double stator consisting of plates 37 connected, respectively, to the conductors 11 and 12 by conductors 38 and 39, and a movable plate 40 connected to ground by the conductor 33. The plate 40 is adjusted until there is no response in the bones, as described above in connection wit the variable resistor 30, by the use of the modified Wagner earth connection, shown in Fig. 3, all stray capacity currents to ground are eliminated, and the accuracy of the instrument is noticeably improved under adverse conditions.

The operation of the meter may be described by the following method.

I,a=kE-(1). The current in the D. G. meter 24 is 1,, and the voltage applied to the input terminals 3 and 4 is E. Both of theseuantities are static values having the depen ence indicated in the above equation (1) E is the independent variable and 1,, is the dependent variable; a and k are, for the purposes of this discussion, considered as constants. If any value is assigned to E, the value of I,,-a is roportional to the uare of the value E. 11 other words, the c ange in plate current is proportional to the square of the voltage applied to the input terminals. It may be well to note that the numerical value of a is determined experimentally by letting E =0 whereupon a=I,. It is also convenient to let (1) I ,,-a= AI, and to neglect in. This last assumption is justified by the fact that, if proper units are chosen, k can be made ual to 1 numerically and, therefore, vanis me from the equation. We will, therefore, consider the simplified equation. (2) AI,=E.

In practice, the D. C. meter 24' is calibrated to read AI, in suitable units."

We have seen that the change in plate current is proportional to the square of the applied voltage. Now, if we start the calibration of the D. C. meter 24 at numerical value equal to a and use the proper units, we will find that the meter reading (A1,) equals the square of the applied voltage, (E)

Since the root-mean-square, or RMS value of a D. O. volta e is equal to its peak value, no further proo is necessary to show that the voltmeter deflection (A1,). is equal to the square of the RMS voltage (E applied.

Suppose the applied voltage is a sine wave having an Rlllgvalue of E... The peak value of this is /215 (see A. G. text books for proof of this). The equation of this wave is E= /2E, sin qt where f[=21rf and f=frequency, 1r=3.1416 I t=time. (This is also proved in nearly all A. text books.)

If E= /2Er sin E =2 sin gt and from trigonometry sin qt= 2 soit follows that E*= (E,)= (B2) cos 2 qt. Then, from (2) meter deflection (A1,) (E,) (1 7,) cos 2 qt. A D. C. meter (DArsonval type) shows a deflection proportional to the average current if it is sufliciently well damped so that it does not follow the variations in the current. (See text books on measurements for further details.)

The average value of any alternating current is zero. (This is also an elementary A. 0. text book proof.) The average value of r? 2 a is The A. C. component (Er) cos 2 qt has no eifect on the D. G. meter reading. Therefore, the deflection (A1,) equals the square of the RMS voltage (E,) applied.

Suppose the a plied voltage is a complex wave made up 0 two sine-wave com nents of different frequencies and a D. com- I ponent.

This is roved in most A. C. text books.)

rom (2 A1,,=2 Em Sim pt+2 (E2): Sim 5+ (E. as before the square of the RMS value of any wave, regardless of its wave form.

Method 2:

(1) I,,a==kE as before which is reducible by the same process as in Method 1 to, the form (2) Al =E The square of the RMS value of a voltage wave is the average of the squared values of,

the instantaneous voltages by definition. I

(This is usually given in text books as E rms 2 rf e da.

Now, the direct-current-meter' deflection varies only as the average of the instantaneous values of plate current and,'since Al,,= E (2), we may say that the meter deflection varies as the average of. the squared instantaneous values of applied voltage. But, since the average of squared instantaneous values of the applied voltage is its EMS value squared. then it follows that the meter deflection varies as thesquare of the RMS value of the voltage applied to the device.

The deflection will be the same when directcurrent is impressed on the grids as when the same RMS value of alternating-current is impressed. It will be noted that the direction of the direct current is not important. The device may then be calibrated on directcurrent and will indicate RMS values of alternating current. This indication will be accurate, regardless of wave form.

Several corrections are desirable but need not be, made unless extreme accuracy desired. However, it is essential that the plate voltages on' the two tubes be adjusted separately to give the same plate current with the same grid bias. It is also best to so tad-- just the input resistors that the plate current will be the same in each tube when directcurrent is ap lied-to the input terminals and reversed. I high accuracfiv is desired, the plate voltage will be adj uste untilthe change in plate current from the initial reading is proportional to the square of the impressed voltage.

A vacuum-tube voltmeter, constructed in accordance with my invention, has the following advantages over prior devices:

The meter indicates true RMS values for the complete cycle since each tube takes care of one-half. Prior meters utilizing only one tube indicate correctly only when the wave is symmetrical; i. e. contains no even harmonics or direct-current.

The meter may be calibrated on directcurrent.

, The meter indicates low voltages of approximately one volt more accurately than many direct-current meters.

The meter is rugged and will stand large overloads without danger of burnout.

The input impedance may be made very high without materially reducing its sensitivity or accuracy.

High-resistance resistors may be inserted in series-circuit relation with the grids to safeguard the meter from extreme overloads because the grid current will provide additional bias. Such expedient does not effect the calibration of the meter at audio frequencies.

The fact that almost perfect squaring ac-.

, tion is obtained is due largely to the summation of two semi-parabolic functions. Ordinarily, such functions become erratic for small values. By summing them up, however, the errors become so" small as to be unnoticeable. I have found it possible to adjust this exponential function over a rather large range of values by changing the plate voltage. Variations in grid voltage have a similar effect on the exponent.

I do not limit my invention to the specific arrangements illustrated, as various modifications may be made without departing from the spirit and scope of the invention, as set forth in the appended claims.

I claim as my invention:

1. In combination a plurality of thermionictubes severally comprising a cathode, an anode and an auxiliary electrode in an evacuated vessel, aineasuring instrument and a source of voltage associated with the anodes andvcathodes of said rectifiers, means to compensate for the diflerence in amplification constants of each of said tubes, and means for applying a voltage to be measured to said auxiliary electrodes.

2. In combination, two thermionic tubes severally comprising a cathode, an anode and an auxiliary electrode in an evacuated vessel. a current-measuring instrument and a battery connected between said anode and said cathode'of each of said tubes, means to compensate for the difference in amplifies: tion constants of each of said tubes, and means for applying voltage to be measured to said auxiliary electrodes.

3. A voltmeter comprising a plurality of vacuum tubes having interconnected input and output circuits, a measuring instrument in the output circuit of said vacuum tubes, means to compensate for the difierence in amplification constants of said tubes, and means for impressing upon the input circuit of sdaid vacuum tubes a voltage to be measure 4. In a system for measuring the rootmean-square value of an alternating voltage, the combination with a measuring instrument, of two triode vacuum tubes connected in electrical-circuit relation in such manner that their plate currents are additive, and means for impressing an alternating volt to be measured on grids of said tubes, said voltage being simultaneously positive for one tube and negative for the other tube.

5. In a vacuum-tube voltage-measuring system, a plurality of vacuum tubes sever ally comprising an anode, a cathode and a grid, an output circuit for said tubes including a battery and a measuring instrument in series-circuit relation with the anode and the cathode of each of said tubes,

an input circuit for said tubes including a N cipal electrodes are additive, and means for impressin an alternating voltage to be measvariable resistor in series-circuit relation with the cathode and the grid of each of said tubes a battery and a variable resistor for energizing said cathodes, and means for impressing a voltage to be measured on the grid circuits of said tubes.

6. In a vacuum-tube voltage-measuring system, the combination with two vacuum tubes each havin an anode, a cathode and an auxiliary electr e, of a battery and a measuring instrument in series-circuit relation with the anode and cathode of each of said tubes, a batter and a variable resistor for energizing sai cathodes, a battery and a variable resistor for biasing the potential of said auxiliary'electrode, and means for impressing a volta to be measured. on the auxiliary electr es of said tubes.

7. In combination, a lurality of vacuum tubes, each having a cat ode, a plate, and a grid, means for conveying a voltage to be measured, means for connecting said voltageconveyin means in series circuit relation with sai ids, means for addingthe plate currents og said tubes, and means for measuring said added plate currents.

8. In combination, a plurality of vacuum tubes, each having a cathode, a plate and a grid, means connecting said cathodes in electrical-circuit relation, an ammeter, means connecting the plate and cathode of each of said tubes in series-circuit relation with said am'meter, means to compensate for the dificrencn in amplification constants of said tubes,

and means for impressing an electromotiveforce to be measured on the grids of said tubes.

9. A tube voltmeter comprising a pluralit of vacuum tubes severally comprising a gri a filament and a plate, a micro-ammeter and a battery connected to the plates of tubes, and means for charging the grids of said tubes with a voltage to be measured, said tubes being adjusted to work on semi-parabolic grid-voltage-plate current characteristics, and said grids being 0 positely charged with respect to each other y the voltage to be measured.

v10. The combination with a plurality of vacuum tubes severally comprising a grid, a plate and a'filament, of means for oppositely charging said grids in accordance with a voltage to measured, means for working said tubes on semi-parabolic grid-voltageplate-current characteristics, and means for measuring the sum of the plate currents of said tubes.

11. In a system for measuring the rootmean-square values of an alternating voltage, the combination with a measuring in- .strument of two plural-electrode electric disof the type incorporating charge devices odes and principal electrodes,

control electr connected in electric-circuit relation in such manner that the currents between their prin- In testimony whereof, I have hereunto snbscribed my name this tenth da of July 1929.

JAMES BOOTH. 

